I have installed Humor Sans.ttf onto the main system, and into the directory: /home/joal/bin/sage-6.2/local/lib/python2.7/site-packages/matplotlib-1.3.1-py2.7-linux-x86_64.egg/matplotlib/mpl-data/fonts/

... but don't know how to reset the matplotlib font cache so matplotlib will pay attention to the new font.

[EDIT]

Okay, seem to have part of the solution - need to verify on a Linux machine this weekend.

EDIT of EDIT
The tricks below don't work on LinuxMint.

The font is recognised in the virtualbox image if it is dumped into:sage-6.2/local/lib/python2.7/site-packages/matplotlib-1.3.1-py2.7-linux-x86_64.egg/matplotlib/mpl-data/fonts/
and renamed Humor-Sans.ttf

Then you need to create a .matplotlib folder in the user directory.mkdir ~/.matplotlib

The concern is that it now that Humor Sans is used for text font, but not for the numbers on the horizontal and vertical axes. Any suggestions, sagers?

Under Linux, the .matplotlib directory lives in ~/.sage. cd into
it and remove the fontList.cache

Currently, use of constant-value tick_formatters such as pi, e,
sqrt(2) will default back to using the normal font - this is unlikely
to have a quick solution as the Humor Sans font is missing several mathematical symbols any way.